Firefox Accounts migration banner (bug 1052864)

https://bugzilla.mozilla.org/show_bug.cgi?id=1052864

This adds custom elements for the site banner (`<mkt-banner>`) and login (`<mkt-login>`).

r? @cvan
`undismissable` vs. `dismiss="persist"`? what's the difference. `dismiss="persist"` seems confusing to me.